Glueballs  Hadron Structure  Hadron Structure

Meson Spectroscopy

The past five years were characterised by an impressive step forward due to the efficient operation of the LEAR machine at CERN and to a co-ordinated use of two advanced wide angle spectrometers for neutral and charged particle detection (Crystal Barrel and Obelix), which started operation in 1990. More than 10⁹ events in $\overline{p}N$ annihilation were collected, about three orders of magnitude larger than previously available from Bubble Chambers. The analysis is far from being complete, but already very high statistics signals for new mesons have been found in $\overline{p}p$ annihilation in several channels. One of these states, f₀(1500) ( $\Gamma _{tot}=120\pm 19$ MeV), has mass and width very much consistent with those anticipated for a glueball by lattice gauge (LGT) theories (Fig. ).   

Figure: Glueball production observed by the Crystal Barrel in $\overline{p}p$ collisions at LEAR. Before operation of LEAR, the world total number of events in $\overline{p}p\rightarrow \pi \pi \pi$ was only a few hundred. Today a million events have been measured, revealing detailed structure in the Dalitz plot. The horizontal band marked X reveals the existence of a scalar meson mass 1500 MeV/c² with properties consistent with the lightest glueball, predicted by lattice gauge theory.

 

It is seen to decay in the channels $\pi \pi$, $4\pi$, $\eta \eta$, $\eta \eta ^{\prime }$ and $K\overline{K}$. The most likely interpretation is that this is the scalar glueball mixed with the J^PC=0⁺⁺ $q\overline{q}$ nonet also expected in this region. Other possible members of this nonet, such as a₀(1450) and f₀(1370), have also been identified at LEAR. The f₀(1500) was previously observed with limited statistics (typically 100 decays or less) in central collisions at the $\Omega$- spectrometer at CERN, by the GAMS Collaboration in peripheral production with $\pi ^{-}$ beams and in $J/\psi$ radiative decay (Mark III and Beijing). The decisive progress achieved at LEAR is mostly due to the enormous statistical samples (typically 10⁵ f₀(1500) decays) made possible by the $4\pi$ acceptance of the detectors, together with substantial refinements in the analysis tools.